a = 1;
b = 1;
c = 1;
f = function(x) a*(exp(1)^(-b*(exp(1)^(-c*x))));

polygon([
    [-10, 0],
    for (x = [-10:0.1:10]) [x, 5 + f(x)],
    for (x = [10:-0.1:-10]) [x, -5 - f(x)],
    [-10, 0],
]);